Outside of sheer athletic performance, the essence of having a “blasting” or explosive double leg takedown is a proper stance.
(If you need some general advice, check out the basics of a double leg takedown here)
To fully load up your stance for a penetrating double leg attack, I have two words: Drop and Drive.

While there are probably hundreds of different styles of the double leg takedown, both of these elements really need to be in place to lunge in quickly for a tackle.
The Drop

The first step is to change your level from a regular mma fighting stance to a lower, wrestling type position. This level change is “the drop”. When you drop, make sure to bend via the knees, not the hips. Above, Denis Kang shows good form with his hips are beneath his shoulders. He’s not bending over at the waist – which will nerf your explosiveness.

Above, you’ll see Tito Ortiz about to shoot into a double leg blast. Note how wide and low his stance is – his loaded stance allows him to push off the back leg and drive forward.
The Drive

Denis Kang finishes his double leg takedown by driving off his back leg, and penetrating below his opponent’s center of gravity. In addition to bursting forward like a runner at the start of a race, DRIVE can also refer to your tenacity in finishing the takedown. You keep pushing and running through an opponent until they are on the mat or cage wall.
Why cant I do both at the same time?

I know its tempting to try to slide down the hypotenuse (that’s the long side) of the triangle drawn above. Do not do this.
The main problem is that by trying to drop as you drive, you don’t do either very well. It opens you up to getting kneed in the face, since your are leading the whole movement with your head. It also make it easier to defend your double leg attempt because your drive isn’t as fast, powerful, or well positioned (ie a good stance to back up your tackling motion).
Below, a small example of why leading with your head (sliding down the triangle) is a very bad idea.

Resources
For fighters looking for a little extra, here are the videos I took the screencaps from, plus a few more for the lavishing’s sake.
Tito Ortiz shows a double leg takedown, etc
http://www.youtube.com/watch?v=TH5_eW5lBmk
Duke Oursler from Redline MMA shows some technical points on the double leg; notice how he drops!
http://www.youtube.com/watch?v=JVi2pzMgt6U
Denis Kang and Steven Kesting show double leg takedowns against the jab – nice details to MMA specific takedowns.
http://www.youtube.com/watch?v=bmJFxkaUI0k
The Utah local Damage Control MMA crew show another take on the double leg takedown.
http://damagecontrolmma.com/wrestlings-jab-the-basic-double-leg-takedown/
Tags: double leg, takedown, wrestling